The present invention relates generally to gas turbine engines, and in particular to a system and method for starting an auxiliary power unit (APU) at high altitudes.
Starting a gas turbine requires the engine to be motored to a certain speed such that the fuel-air mixture provided to the combustor is conducive to light-off (i.e., successful ignition of the engine). These conditions become much more difficult to achieve when starting the engine at high altitude. For example, if an aircraft is flying at 40,000 feet, there will be very little air flowing through the engine, and the air that is flowing through the engine will be traveling at fairly high velocities. This makes it very difficult to start an APU, or any gas turbine engine at high altitudes.
Traditionally, starting an APU at high altitude involves spooling the engine up to a constant start speed. When the engine is up to speed, a constant amount of fuel is provided to the combustion chamber for lighting. An igniter in the chamber continuously sparks to attempt to light-off the engine. Due to the unstable air conditions at high altitude, this process is very unpredictable and can cause serious delays in startup of the APU. It is desirable to provide a faster, more reliable method of starting a gas turbine engine at high altitude.